Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
An ion implanter includes an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam typically is mass analyzed to eliminate undesired ion species, accelerated to a desired energy, and implanted into a target. The ion beam may be distributed over the target area by electrostatic or magnetic beam scanning, by target movement, or by a combination of beam scanning and target movement. The ion beam may be a spot beam or a ribbon beam having a long dimension and a short dimension.
Turning to FIG. 1, a block diagram of a beam-line ion implanter 200 that may provide ions for doping a selected material is illustrated. Those skilled in the art will recognize that the beam-line ion implanter 200 is only one of many examples of beam-line ion implanters that can provide ions for doping a selected material.
In general, the beam-line ion implanter 200 includes an ion source 280 to generate ions that form an ion beam 281. The ion source 280 may include an ion chamber 283 and a gas box containing a gas to be ionized. The gas is supplied to the ion chamber 283 where it is ionized. Ions are created by applying a voltage across the electrodes of the chamber, known as an arc voltage. Additionally, a magnetic field is provided to control the motion of the ionized particles. This is achieved by passing a current through a source magnet. This gas may be or may include, in some embodiments, arsenic, boron, phosphorus, carborane C2B10H12, or another large molecular compound. In other embodiments, the gas may be an alkane, such as ethane, or another atomic or molecular carbon-containing species. The ions thus formed are extracted from the ion chamber 283 to form the ion beam 281. The ion beam 281 is directed between the poles of resolving magnet 282. A power supply is connected to an extraction electrode of the ion source 280 and provides an adjustable voltage, for example, between about 0.2 and 80 kV in a high current ion implanter. Thus, singly charged ions from the ion source are accelerated to energies of about 0.2 to 80 keV by this adjustable voltage.
The ion beam 281 passes through a suppression electrode 284 and ground electrode 285 to mass analyzer 286. Mass analyzer 286 includes resolving magnet 282 and masking electrode 288 having resolving aperture 289. Resolving magnet 282 deflects ions in the ion beam 281 such that ions of a desired ion species pass through the resolving aperture 289. Undesired ion species do not pass through the resolving aperture 289, but are blocked by the masking electrode 288. In one embodiment, resolving magnet 282 deflects ions of the desired species by about 90°.
Ions of the desired ion species pass through the resolving aperture 289 to the angle corrector magnet 294. Angle corrector magnet 294 deflects ions of the desired ion species and converts the ion beam from a diverging ion beam to ribbon ion beam 212, which has substantially parallel ion trajectories. In one embodiment, the angle corrector magnet 294 deflects ions of the desired ion species by about 70°. The beam-line ion implanter 200 may further include acceleration or deceleration units in some embodiments.
An end station 211 supports one or more workpieces, such as workpiece 138, in the path of ribbon ion beam 212 such that ions of the desired species are implanted into workpiece 138. The end station 211 may include a platen 295 to support the workpiece 138. The end station 211 also may include a scanner (not shown) for moving the workpiece 138 perpendicular to the long dimension of the ribbon ion beam 212 cross-section, thereby distributing ions over the entire surface of workpiece 138. Although the ribbon ion beam 212 is illustrated, other embodiments may provide a spot beam.
The ion implanter may include additional components known to those skilled in the art. For example, the end station 211 typically includes automated workpiece handling equipment for introducing workpieces into the beam-line ion implanter 200 and for removing workpieces after ion implantation. The end station 211 also may include a dose measuring system, an electron flood gun, or other known components. It will be understood to those skilled in the art that the entire path traversed by the ion beam is evacuated during ion implantation. The beam-line ion implanter 200 may incorporate hot or cold implantation of ions in some embodiments.
Ion implantation is an effective method to introduce dopants into a substrate, however there are unwanted side effects that must be addressed. For example, implanted ions often distribute themselves at deeper depths than expected. It is believed that this is caused by a phenomenon known as channeling, where ions are moved or channeled along axes and planes of symmetry in the crystalline structure. Thus, the ions may be implanted substantially between atoms in the crystal lattice in the substrate. This channeling effect causes a deeper concentration of the dopant, which increases the effective junction depth.
Traditionally, to overcome this problem, the workpiece or substrate is implanted with heavier species before the actual dopant implantation. This implantation is known as the pre-amorphization implantation, or PAI. Typically, a heavier species, such as silicon or germanium is implanted into the substrate to effectively change the silicon crystalline structure into an amorphous layer. Because the amorphous layer lacks an organized crystal structure, the implanted ions may not channel between atoms in the crystal lattice. This amorphous layer significantly reduces channeling, thereby alleviating the issue described above.
However, the PAI step is not without its drawbacks. These species tend to cause residual damage at end of range (referred to as EOR defects). For example, germanium creates a large amount of damage, in terms of dislocation. Furthermore, germanium does not recrystallize well during the annealing process. These EOR defects introduce leakage into the resulting CMOS transistors. As junction depths get smaller and smaller, this leakage becomes more problematic.
In certain embodiments, carbon can be used as a co-implant species in association with another PAI, typically germanium. The purpose of the carbon implant is to position the carbon between the shallow dopant and the EOR damage caused by the PAI implant. These implanted carbon atoms, also known as substitutional carbon, may block some of the interstitials coming back from EOR during the anneal that would otherwise cause transient enhanced diffusion (TED) and boron interstitial cluster (BIC) formation. However, the range of carbon often overlaps with that of the PAI species, and so the carbon implant itself contributes to PAI. Thus, carbon may also be used as a PAI species in its own right.
Carbon can also be used to create localized compressive strain. Therefore, if one creates a source/drain out of SiC, carbon implantation will cause tensile strain in the channel. This may improve NMOS behavior. Incorporating carbon into the crystal lattice may require the use of epitaxial growth or the implantation a high dose of carbon into the silicon lattice. This may cause amorphization, and in regrowth the carbon is incorporated into the lattice.
Amorphization and stress are both important to semiconductor manufacturers. Accordingly, there is a need in the art for a new and improved method of implanting a carbon-containing species, and more particularly, a new and improved method of implanting ethane.